1. Technical Field
The present application relates generally to an electrophotographic imaging device and more particularly to a fuser for an electrophotographic imaging device.
2. Description of the Related Art
In the electrophotographic (EP) imaging process used in printers, copiers and the like, a photosensitive member, such as a photoconductive drum or belt, is uniformly charged over an outer surface. An electrostatic latent image is formed by selectively exposing the uniformly charged surface of the photosensitive member. Toner particles are applied to the electrostatic latent image and thereafter the toner image is transferred to the media intended to receive the image. The toner is fixed to the media by a combination of heat and pressure applied by a fuser.
The fuser may include a belt fuser that includes a fusing belt and an opposing backup member, such as a backup roll. The belt and the backup member form a nip therebetween. The media with the toner image is moved through the nip to fuse the toner to the media. Belt fusers allow for “instant-on” fusing where the fuser has a relatively short warm up time thereby reducing electricity consumption. Fusing speed is a function of the width of the fuser nip and the belt surface temperature, among other things. A fuser with a relatively wide nip is able to fuse toner to media moving at higher speeds through the nip than a comparable fuser with a relatively narrow nip. Further, a fuser with a higher belt surface temperature is able to fuse toner to the media faster than a fuser with a lower belt surface temperature. Higher fusing speeds in turn lead to higher print speeds.
Fusers in laser printers are designed to bond toner to the entire width of media by using heat and pressure. In most fusers, heat is generated by either by a halogen lamp or a ceramic heater. In the case of the halogen lamp fuser, heat is transferred radiantly from the lamp to the black coated inside of an aluminum tube. For monochrome printers, the aluminum tube may have a release layer of either a perfluoroalkoxy (PFA) or polytetrafluoroethylene (PTFE) coating. For color printers, the aluminum tube may be first coated with silicone rubber and then a perfluoroalkoxy (PFA) sleeve. In the cases of the fuser with ceramic heater, heat is transferred conductively from the ceramic heater to either a polyimide tube with a PFA and/or PTFE release layer (for a monochrome fuser), a stainless steel tube with a PFA and/or PTFE release layer (also a monochrome fuser), or a stainless steel tube with a silicone layer and a PFA sleeve (for a color fuser). The release layer coated surface of these tubes applies the heat to the surface of the media that has toner. The pressure is produced by a rubber coated steel or aluminum shaft that is pressed against the coated tube. The media passes between the coated tube and the rubber coated steel shaft. The rubber coated steel shaft typically has a PFA sleeve placed over the rubber coating. This rubber coated steel shaft is commonly called a backup roll. The length of the heating region is typically about 2 to 3 mm longer than the widest media that the laser printer is designed to print. An overheating problem occurs when narrow media is printed in the laser printer. In regions of the fuser nip where the media does not pass through the fuser, the tube and backup roll become very hot and may be damaged due to the high temperature.
In particular, in this case heat generated by the ceramic heater is not removed from those regions of the fuser nip which fail to contact media sheets passing through the nip. The heat generated in such regions heats the tube and the backup roll as a result. Because laser printers are designed to have a very small first copy time, the thermal mass of the heater and of the tube is very small. Because of the small thermal mass, the axial heat conduction from hot regions of the tube and heater to cooler regions is very small. This causes the amount of heat to build up relatively rapidly in the heater and tube in such fuser nip regions not contacting the passing media sheets. The heat build up is not significant for fuser nip regions contacting the media sheets because energy is removed from the system by the sheets and toner fixing. In addition, to achieve the very small first copy time and fix the toner to the media, the backup roll surface needs to become very hot without conducting heat to the steel or aluminum shaft. This is achieved because the rubber is a thermal insulator. However, this also means the heat conducted away from the coated tube and heater by the backup roll in regions not contacting the media sheets is very small.
One other possible mechanism to remove heat from the coated tube and backup roll in such overheating fuser nip regions is by convection into the air. Unfortunately, the amount of heat removed by convection is very small because in order to meet the very small first copy time, the heat lost to the air is minimized by enclosing the coated tube and backup roll in plastic covers to keep the air still. However, the plastic covers are designed to act as a heat insulating surface, thereby providing little if any opportunity to reduce heat to the overheating fuser nip regions via air convection.
A current solution to prevent overheating is to reduce the velocity of the media sheets traveling through the laser printer and increase the distance between media sheets. Reducing the velocity of the media allows the temperature of the coated tube to be reduced. The reduced temperature produces less heat in the overheating regions. The distance between sheets is increased so that, with the reduced media velocity, the time between media sheets becomes large enough for the small heat conduction to cool the overheating regions and prevent overheating. As the media widths become smaller, the amount of time needed to cool the overheating fuser nip regions becomes larger because the size of such regions becomes larger. The overall result of increasing sheet spacing between narrow media is that narrow width media is printed very slowly. For example, existing laser printers may reduce printing speeds for some media sheets of a print job more than 50%.
Accordingly, it will be appreciated that an efficient belt fuser with enhanced heating performance is desired.